We have demonstrated a requirement for cellular ATP in the receptor-mediated endocytosis of transferrin. This has been accomplished using a novel assay for endocytosis based on acquisition of resistance to the membrane impermeable reducing agent, glutathione (GSH). Diferric-transferrin was conjugated to biotin via a cleavable disulfide bond and iodinated. Internalization of 125I-biotin-S-S-transferrin (125I-BSST) was quantitated by adsorption to avidin-Sepharose after treatment of cells with GSH. Receptor-mediated endocytosis of 125I-BSST was severely inhibited in ATP-depleted cells. Similar results were obtained when ATP was depleted by incubation of cells either under a N2-atmosphere or in the presence of NaN3 and NaF. The latter treatment, alone, also resulted in a loss of surface transferrin receptors which could not be correlated to reductions in cellular ATP. In contrast to the acquisition of GSH resistance, the apparent internalization of 125I-BSST as assessed by inaccessibility to antitransferrin antibodies reached control levels in ATP-depleted cells. Our biochemical and morphological data suggested that, although ATP is required for receptor-mediated endocytosis, in ATP-depleted cells ligands can become efficiently sequestered into deeply invaginated pits that are inaccessible to large probes such as antibodies, but remain accessible to small molecules such as GSH.

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